Process for manufacturing hollow plastic objects

ABSTRACT

A process for manufacturing hollow synthetic resin bodies or objects in which cores are produced in casting molds, then placed into molds of injection molding machines, and subsequently enveloped with injected synthetic resin material ( 35 ) which then solidifies. The molds are then heated up to a temperature at which the cores melt and at which the molten cores can be removed from the molds. The cores are made of shell halves ( 32, 33 ) which are assembled before the synthetic resin material ( 35 ) is injection molded around them.

BACKGROUND OF THE INVENTION

The invention relates to a process for manufacturing objects ofsynthetic resin.

In FR-PS 1 366 921 a process is disclosed for the manufacture ofthermoplastic parts made by injection molding with at least one openingto a hollow cavity. For this purpose the molten thermoplastic materialis injected into a mold which consists of strong material of goodthermal conduction, and in which at least one core is provided whichconsists of a hard material and is resistant to collapse and deformationduring injection molding, but begins to melt at a temperature that isbetween the deformation temperature and the resoftening temperature ofthe thermoplastic material. After injection molding the shaped objectwith the core is heated to a temperature between the deformationtemperature and the softening temperature of the shaped object so thatthe core will melt out without melting the thermoplastic material, inorder thus to remove the core from the casting. An alloy of bismuth andtin is named as the material for the remeltable cores. After meltingout, the metal is usable again and is supplied again to the castingprocess.

Furthermore, DE-OS 26 28 390 discloses a method and an apparatus forproducing one or more hollow sand cores for casting molds. Theseso-called shell cores are made by mixing thermosetting binding agentwith sand. This sand is shaken in a heated core box and after the coresurface has hardened the entire core box is rotated 180° and the excesssand that has not hardened is poured out. In this manner hollow coresare obtained which are more economical than solid cores due to thelesser amount of material and their quick preparation.

A disadvantage of the known state of the art is that, for one thing, asolid core requires the use of a large amount of material, but on theother hand the use of hollow cores, especially when metal is used as thecore material, is complicated and at the same time there is no assurancethat a uniform wall thickness will be maintained.

SUMMARY OF THE INVENTION

The invention is addressed to the problem of avoiding the aforementioneddisadvantages and providing a process for manufacturing objects ofsynthetic resin which while using a small amount of material, willpermit great accuracy of the cores and hence of the component beingmade.

This problem is solved by the invention as described and claimedhereinafter.

A process for manufacturing a hollow synthetic resin product is knownfrom DE 41 21 441 in which half-shells or halves of a core are made ofwater-soluble resin. These halves, however, must additionally be sprayedwith a water-resistant resin. After the hollow plastic product is madethe non-water resistant resin is washed out, while the water-resistantresin remains in the synthetic resin product.

In contrast thereto, the advantage of the invention is that the partialshells of metal permit a seamless joining together and additionalelements or additional spraying or wrapping are not necessary.

According to one embodiment of the invention the partial shells arepressed or squeezed together. It is also possible to affix the partialshells together via an undercut in the manner of a snap fastening. Ofcourse, the metal of the partial shells can also be welded or cementedto each other.

If desired, the partial shells can be sprayed or coated with a lacqueror a hardenable liquid after assembly. The purpose of this layer is toimprove the strength or the surface of the synthetic resin object afterthe core has been melted out of the synthetic resin object.

If the half shells or partial shells have a very large hollow interiorcavity, it is possible to provide support by means of an internalstructure. This structure is composed, of course, of the material of thecores and is removed from the hollow synthetic resin object when thecores are melted.

These and additional features of preferred embodiments of the inventionwill be found not only in the claims but also in the description and thedrawings, and the individual features can each be realized individuallyor together in the form of subcombinations in embodiments of theinvention and in other fields, and may constitute advantageous as wellas independently patentable embodiments, for which protection is herebyclaimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described below with reference to a working embodimentshown in the accompanying drawings.

FIG. 1 shows a motor intake tube which was manufactured by the coremelting technique,

FIG. 2 shows a core put together from a plurality of half shells,

FIG. 3 shows a cross section through a half-shell core,

FIG. 4 shows a tool for producing the half shells.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A motor intake tube or suction tube 10 for an internal combustion engineis produced from a thermoplastic synthetic resin. The intake tube 10 iscomprised of an air collecting chamber 11 to which clean air filtered bya filter element, not shown here, is delivered. From this air collectingchamber 11 individual intake tubes 12, 13, 14, 15, 16, 17 extend to aconnecting flange 18 joining the individual intake tubes together. Inthe connecting flange there are openings 19, 20, 21, 22, 23, 24 forinjection nozzles, as well as mounting holes.

The process steps in the manufacture of such an intake tube are themanufacture of the core in a low-pressure process from a tin-bismuthalloy. This core is taken from the casting mold and placed in aninjection molding machine. After thermoplastic synthetic resin materialhas been injected all around the core, the core is melted out and thesynthetic resin object is washed out. A core of this type for themanufacture of an intake manifold comprising six individual intake tubesweighs between 40 and 50 kilograms; this of course makes handling itconsiderably more difficult.

FIG. 2 shows a core for the intake manifold shown in FIG. 1. It iscomprised of six tubes 25, 26, 27, 28, 29, 30 which are constructed bythe half-shell technique, as well as a collecting tube 3 1. The tubes 25to 30 are joined to the manifold tube 31 by plug-in connections.

The construction of the tubes is shown in FIG. 3. They consist of anupper half shell 32 and a lower half shell 33, which are interlocked orsnap-fastened to each other, for example, in connecting region 34. Thetubes are, as already mentioned, sprayed all around with a thermoplasticsynthetic resin material 35. This forms the intake tube or theindividual tubes according to FIG. 1. The half shells can be madeaccording to their geometry in a core casting machine, since themachines can be opened on two sides and a stage tool can be used withoutadditional difficulty.

In particular it is possible to carry out the pressing of the halfshells in the core casting mold. For this purpose the center plate 39 ofthe mold is removed and the two outer mold halves 37 and 38 are closed,so that the half shells 40 and 41 can be assembled to one another. Thusthere is no need for an additional seam apparatus.

An important advantage in the use of tubes as cores also is that, whenthe core is melted out the melting liquid can get into the core and thusresults in a more rapid heat transfer and a faster melt-out.

What is claimed is:
 1. A process for the manufacture of synthetic resinobjects comprising the steps of producing at least one hollow core bycasting a plurality of metal half shells in a casting mold comprising apair of opposed mold halves which close on opposite sides of a centerplate, and after casting said cast metal half shells, the center plateis withdrawn from between the mold halves and said cast metal halfshells are assembled and joined together by reclosing the mold halvessuch that said cast metal half shells are pressed in contact with eachother to form said at least one core such that said cast metal halfshells enclose a hollow interior cavity, placing the produced at leastone hollow core in a mold of an injection molding machine, injectingsynthetic resin material around said at least one core in said mold ofthe injection molding machine so that the at least one core is enclosedin the synthetic resin material, said synthetic resin material having amelting temperature which is higher than the melting point of the core,and thereafter heating said at least one core enclosed in syntheticresin material to a temperature at which said at least one core melts,and removing the molten material from the mold of the injection moldingmachine.
 2. A process according to claim , wherein the half shells arepressed or squeezed together.
 3. A process according to claim 1, whereinthe half shells are assembled by affixing them to each other via anundercut by a snap fastening.
 4. A process according to claim 1, whereinthe half shells are welded or cemented to one another.
 5. A processaccording to claim 1, further comprising the step of spraying or coatingthe assembled half shells with a lacquer or a hardenable liquid prior toplacing the at least one hollow core in the mold of the injectionmolding machine.
 6. A process according to claim 1, wherein the hollowcore further comprises an internal structure for supporting theassembled half shells.